The invention relates to a heat-resistant insulated electric wire and a cable composed of a plurality of such wires. More specifically, the invention relates to a heat resistant insulated electric wire which comprises an electric conductor and a coating layer thereon of a mixture of a binder and a finely ground and unmelted inorganic substance. This mixture is adhered onto the said electric conductor by heating it at a temperature below its melting point; then an organic insulating layer is formed on the said layer of the mixture. The heat-resistant insulated electric wire of the invention is an excellent heat-resistant insulated electric wire having superior durability, suitable for use as a structural component in magnet-motors which have a structural component consisting of an electric wire coil which is very tightly wound (under a constrainted condition), for example as in the wiper motor of an automobile. Such purposes require heat-resistance and non-flammability.
Heretofore such a heat-resistant insulated electric wire has been produced by baking on a conductor a heat-resistant resin such as polyimide, polyamidoimide, polybenzimidazol, polyimidazolopine, polyesterimide and the like. In recent years, with increased demand needs for small size and weight lightening together with high power, several kinds of ceramic insulated electric wires have been developed. Importance has been attached to the use of this type of electric wire for space exploration and for the interior of a radial ray emitting environment. Furthermore in the field of conductor materials, heat-resistant copper alloys and dispersed reinforced alloys have been developed. Moreover, an almite wire coated with aluminum oxide as an insulator has also been under production.
However, normal use of those almite electric wires having a heat-resistant resin coating as the insulating material thereon are restricted to temperatures below 220.degree. C. for the reason that those resins are organic materials which are weak against heat. Because almite is an inorganic material the normal use of even almite is restricted to temperatures below 300.degree. to 350.degree. C. Accordingly, in order that a heat-resistant insulated electric wire can be made for use in a temperature range greater than that mentioned above, it is necessary to coat the insulated wire with an inorganic material having high heat-resistance, such as glass or ceramics and the like, as an insulating material. Such electric wires have been described in some publications.
Those inorganic material coated insulated electric wires have been produced by baking a heat-resistant resin mixture with powdered mica, clay, talc, glass, short fiber silica and the like contained therein. Recently insulating materials containing as their main insulating component SiO.sub.2 --B.sub.2 O.sub.3 --PbO-glass or SiO.sub.2 --MgO--Al.sub.2 O.sub.3 -glass have attracted much attention in this field. In spite of the merit of excellent heat-resistance, the use of an inorganic material insulated electric wire presents problems due to its lack of adherence to the conductor and its lack of flexibility for coil-winding. Those shortcomings have heretofore severely limited its marketability. Meanwhile, an inorganic material coated insulating electric wire having an excellent heat-resistance has been developed. This conductor is coated by nickel-plating or by applying a clad thereon. The inorganic material coated wire offers an advantage in the fact that an oxidized film does not form on the wire even at a temperature over 600.degree. C. while showing excellent heat-resistance as compared with a steel wire which increases its resistance caused by oxidization at a very high temperature. Under such circumstances, the inorganic material insulated electric wire has again attracted much attention. Nevertheless, the defects of inorganic materials for insulation mentioned above, i.e. the lack of adherence to the conductor and the lack of flexibility necessary for coil winding, have not been solved yet.
Particularly, when insulated electric wires ae used as parts of motors, the feasibility of forming a secure joint with a commutator is the most serious problem.
The connection of a commutator with one end of an electric wire coil is ordinarily carried out in the following manner. That is, one end part of the electric wire coil is wound and caulked on a plural number of clicks of a commutator by an automatic machine, then an electric current is passed through the coil, thereby pushing an electrode toward the clicks from the surrounding area of the commutator. Consequently, the clicks are heated by the generated Joule's heat. An insulating material coated on the electric wire coil is split or melted by the Joule's heat and the conductor of the electric wire coil is connected with and joined to the clicks by the welding process caused by the Joule's heat and the pressure provided by the electrode. Such a connecting method is called "electric fusing". Under these circumstances, a heat-resistant insulated electric wire having the characteristics which make it suitable for "electric fusing" in addition to the properties mentioned above has been eagerly sought by those in this field.
Accordingly, the object of the present invention is to provide a heat-resistant insulated electric wire having thereon an insulating layer superior in flexibility, having excellent heat-resitance and, in addition, having high durability which enables it to tolerate a high-speed winding process. Further such a wire must be able to retain a stable insulation property even after having been subjected to high-speed winding.
Another object of the invention is to provide a process for providing the above heat-resistant insulated electric wire.